High School Earth Science/The Universe

So far we have talked about bigger and bigger systems, from stars to star systems to star clusters and galaxies. The universe contains all these systems, including all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe also includes all of space and time.

Our understanding of the universe has changed a lot over time. The ancient Greeks thought the universe contained only Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. Most people had this basic idea of the universe for centuries, until Galileo first used a telescope to look at the stars. Then people realized that Earth is not the center of the universe, and there are many more stars than thought before. Even as recently as the early 1900s, some scientists still thought the universe was no larger than the Milky Way Galaxy.

Figure 26.18: Edwin Hubble used the 100-inch reflecting telescope at the Mount Wilson Observatory in California to show that some distant specks of light seen through telescopes are actually other galaxies. He also measured these distances to hundreds of galaxies, and discovered that the universe is expanding.

In the early 20th century, an astronomer named Edwin Hubble (Figure 26.18) discovered that the Andromeda Nebula is actually over 2 million light years away—many times farther than the farthest distances we had measured before. He realized that many of the objects astronomers called nebulas were not clouds of gas, but collections of millions or billions of stars—what we now call galaxies. Our view of the universe changed again—we now knew that the universe was much larger than our own galaxy. Today, we know that the universe contains about a hundred billion galaxies—about the same number of galaxies as there are stars in the Milky Way Galaxy.

After discovering that there are galaxies outside our own, Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show us how the universe is changing, and even give us clues as to how the universe formed.

If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. Interestingly, the spectrum will have specific dark bands where elements in the star absorbed light of certain energies. By examining the arrangement of these dark absorption lines, astronomer can actually determine which elements are in a distant star. In fact, the element helium was first discovered in our Sun—not on Earth—by analyzing the absorption lines in the spectrum of the Sun.

When astronomers started to study the spectrum of light from distant galaxies, they noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, as shown in Figure 26.19. This shift of absorption bands toward the red end of the spectrum is known as redshift.

Figure 26.19: Redshift is a shift in absorption bands toward the red end of the spectrum. Redshift occurs when the light source is moving away from you or when the space between you and the source is stretched.

Redshift occurs when the source of light is moving away from the observer. So when astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth. The strange part is that almost every galaxy in the universe has a redshift, which means that almost every galaxy is moving away from us.

An analogy to redshift is the noise a siren makes as it passes by you. You may have noticed that an ambulance lowers the pitch of its siren after it passes you. The sound waves shift towards a lower pitch when the ambulance speeds away from you. Though redshift involves light instead of sound, a similar principle operates in both situations.

Edwin Hubble combined his measurements of the distances to galaxies with other astronomers' measurements of redshift. He noticed a relationship, which is now called Hubble's Law: The farther away a galaxy is, the faster it is moving away from us. In other words, the universe is expanding!

Figure 26.20 shows a simplified diagram of the expansion of the universe. Another way to picture this is to imagine a balloon covered with tiny dots. Each dot represents a galaxy. When you inflate the balloon, the dots slowly move away from each other because the rubber stretches in the space between them. If it were a giant balloon and you were standing on one of the dots, you would see the other dots moving away from you. Not only that, but dots farther away from you on the balloon would move away faster than dots nearby.

Figure 26.20: This is a simplified diagram of the expansion of the universe over time. Note that the distance between galaxies gets bigger as you go forward in time, but the size of each galaxy stays about the same.

An inflating balloon is not exactly like the expanding universe. For one thing, the surface of a balloon has only two dimensions, while space has three dimensions. But it is true that space itself is stretching out between galaxies like the rubber stretches when a balloon is inflated. This stretching of space, which causes the distance between galaxies to increase, is what astronomers mean by the expansion of the universe.

One other difference between the universe and our balloon model involves the actual size of the galaxies. On the inflating balloon, the dots you made will become larger in size as you inflate it. In our universe, however, the galaxies stay the same size; it is just the space between the galaxies that increases as the universe expands.

The discovery that the universe is expanding also told astronomers something about how the universe might have formed. Before this discovery, there were many ideas about the universe, most of them thinking of the universe as constant. Once scientists learned that the universe is expanding, the next logical thought is that at one time it had to have been smaller.

The Big Bang theory is the most widely accepted scientific explanation of how the universe formed. To understand this theory, start by picturing the universe expanding steadily. Then, reverse the direction of time, like pressing the "rewind" button on a video player. Now the universe is contracting, getting smaller and smaller. If you go far enough back in time, you will reach a point when the universe was squeezed into a very small volume.

According to the Big Bang theory, the universe began about 13.7 billion years ago, when everything in the universe was squeezed into a very small volume, as described above. There was an enormous explosion—a big bang—which caused the universe to start expanding rapidly. All the matter and energy in the universe—and even space itself—came out of this explosion.

In the first few moments after the Big Bang, the universe was extremely hot and dense. As the universe expanded, it became less dense and it cooled. After only a few seconds, the universe had cooled enough that protons, neutrons, and electrons could form. After a few minutes, hydrogen could form and the energy in the universe was great enough to allow for nuclear fusion, creating helium atoms in the same way we learned that a star can make helium out of hydrogen atoms, even though there were no stars at this point in the universe's history. The first neutral atoms with neutrons, protons, and electrons, did not form until about 380,000 years after the big bang.

The matter in the early universe was not smoothly distributed across space. Some parts of the universe were more dense than others. These clumps of matter were held close together by gravity. Eventually, these clumps became the gas clouds, stars, galaxies, and other structures that we see in the universe today.

The Big Bang theory is still the best scientific model we have for explaining the formation of the universe. However, recent discoveries in astronomy have shaken up our understanding of the universe. Astronomers and other scientists are now wrestling with some big unanswered questions about what the universe is made of and why it is expanding like it is.

Most of the things we see out in space are objects that emit light, such as stars or glowing gases. When we see other galaxies, we are seeing the glowing stars or gases in that galaxy. However, scientists think that matter that emits light only makes up a small part of the matter in the universe. The rest of the matter is called dark matter.

Because dark matter doesn't emit light, we can't observe it directly. However, we know it is there because its gravity affects the motion of objects around it. For example, when astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot of extra matter in a galaxy that we cannot see.

So what is dark matter? Actually, we don't really know. One possibility is that it could just be ordinary matter—protons, neutrons, and electrons, like what makes up the Earth and all the matter around us. The universe could contain lots of objects that don't have enough mass to glow on their own, such as large planets and brown dwarfs, objects larger than Jupiter but smaller than the smallest stars. Or, there could be large numbers of undetected black holes.

Another possibility is that the universe contains a lot of matter that is unlike anything we have ever encountered. For example, scientists have proposed that there might be particles that have mass but don't interact much with other matter. Scientists call these theoretical particles WIMPs, which stands for Weakly Interactive Massive Particles. WIMPs would have a gravitational effect on other matter because of their mass. But because they don't interact much with ordinary matter, they would be very difficult or impossible to detect directly.

Most scientists who study dark matter believe that the universe's dark matter is a combination of ordinary matter and some kind of exotic matter that we haven't discovered yet. Most scientists also think that ordinary matter is much less than half of the total matter in the universe. Researching dark matter is clearly an active area of scientific research, and astronomers' knowledge about dark matter changing rapidly.

Astronomers who study the expansion of the universe are interested in finding out just how fast the universe is expanding. For years, the big question was whether the universe was expanding fast enough to overcome the attractive pull of gravity. If yes, then the universe would expand forever, although the expansion would slow down over time. If no, then the universe would someday start to contract, and eventually would get squeezed together in a big crunch, the opposite of the Big Bang.

Recently, however, these astronomers have made a strange discovery: the rate at which the universe is expanding is actually increasing. In other words, the universe is expanding faster now than ever before, and in the future it will expand even faster! This answers the old question: the universe will keep expanding forever. But it also proposes a perplexing new question: what is causing the expansion of the universe to accelerate?

One possible hypothesis involves a new, as-yet-undiscovered form of energy called dark energy. We know even less about dark energy than we know about dark matter. However, some scientists believe that dark energy makes up more than half the total content of the universe. Other scientists have other hypotheses about why the universe is continuing to expand; the causes of the universe's expansion is another unanswered question that scientists are researching.

The expansion of the universe is sometimes modeled using a balloon with dots marked on it, as described earlier in the lesson. In what ways is this a good model, and it what ways does it not correctly represent the expanding universe? Can you think of a different way to model the expansion of the universe?

The Big Bang theory is currently the most widely accepted scientific theory for how the universe formed. What is another explanation of how the universe could have formed? Is your explanation one that a scientist would accept?